Abstract

Photosynthesis is the process that enables higher plants, algae and a broad class of bacteria to transform light energy and
store it in the form of energy‐rich organic molecules. In plants and algae, as well as some species of bacteria, photosynthesis
removes carbon dioxide from the atmosphere, produces the molecular oxygen we breath and stores energy in biomass. In addition
some bacteria use light energy to create energy‐rich molecules, but do not split water to produce oxygen. Photosynthesis is
finely regulated to avoid damage caused by excess solar energy. At the same time, though, this regulation also decreases the
efficiency of photosynthesis. Current research is aimed at understanding these responses to improve photosynthetic efficiency,
thus increasing the production of food and fuel.

Key Concepts:

Photosynthesis is the transformation of light energy to chemical energy by higher plants, algae and certain bacteria.

In higher plants, the initial capture of light energy and conversion to the stable, high‐energy products of NADPH and ATP
occurs within and around the thylakoid membranes of the chloroplast. The NADPH and ATP produced are used by enzymes in the
stroma to ‘fix’ carbon dioxide and produce carbohydrates.

Although photosynthesis is driven by light, the photosynthetic apparatus must protect itself from excess solar energy which
causes a decrease in photosynthetic efficiency.

Schematic drawing showing part of a chloroplast. The thylakoid membrane contains the major protein complexes of the photosynthetic machinery responsible for light absorption and electron and proton transfer. The reactions of the thylakoid membrane drive the C3 photosynthetic carbon reduction cycle that takes place in the chloroplast stroma. Illustrated is the concept of light‐driven linear electron flow coupled with the accumulation of protons in the thylakoid lumen, which is in turn used to drive ATP formation by the ATP synthase. In addition to the energy stored in ATP formation, energy derived from absorbed light is stored by the reactions of the thylakoid membrane in the formation of NADPH. Photosynthetic carbon reduction is shown as a three‐stage cycle. (1) Carboxylation: a molecule of carbon dioxide is covalently linked to a carbon skeleton. (2) Reduction: energy in the form of NADPH and ATP is used to form simple carbohydrate. (3) Regeneration: energy in the form of ATP is used to regenerate the carbon skeleton for carboxylation. Key: PS II, photosystem II; PS I, photosystem I; PQ and PQH2, plastoquinone and reduced plastoquinone; cyt, cytochrome; FeS, Rieske iron–sulfur protein; PC, plastocyanin; Fd, ferredoxin; FNR, ferredoxin‐NADP reductase.

Figure 2.

Schematic drawing of the photosystem II reaction centre in the thylakoid membrane. Photosystem II uses light energy to remove electrons from water, resulting in the release of oxygen and protons. The electrons from water are transferred via redox cofactors in the protein complex to form reduced plastoquinone. Key: (Mn)4, manganese cluster involved in removing electrons from water; P680, reaction centre chlorophyll of photosystem II; Pheo, pheophytin; QA and QB, plastoquinones that bind and unbind from photosystem II; YZ, a tyrosine residue in photosystem II that serves as an electron carrier. Figure designed by Michael McConnell, Washington State University.

Figure 3.

The C4 photosynthetic carbon metabolism pathway suppresses photorespiration by concentrating carbon dioxide at the site of carboxylation by Rubisco. The C4 pathway involves two different cell types, mesophyll cells and bundle sheath cells. Shown in the C4 cycle are: carboxylation of carbon dioxide into a four‐carbon acid in mesophyll cells; transport of the four‐carbon acid into bundle sheath cells; decarboxylation of the four‐carbon acid producing a high concentration of carbon dioxide within bundle sheath cells where the C3 cycle produces carbohydrate; transport of the resulting three‐carbon acid back to the mesophyll cell; and the regeneration of the carbon dioxide acceptor, PEP.